CVE-2025-9891 Overview
CVE-2025-9891 is a Cross-Site Request Forgery (CSRF) vulnerability in the User Sync – Remote User Sync plugin for WordPress. The flaw affects all versions up to and including 1.0.2. The mo_user_sync_form_handler() function lacks proper nonce validation, allowing unauthenticated attackers to deactivate the plugin by tricking an authenticated administrator into clicking a malicious link. The vulnerability is classified under CWE-352 and requires user interaction to succeed.
Critical Impact
Unauthenticated attackers can deactivate the User Sync plugin on affected WordPress sites through forged requests, potentially disrupting user synchronization workflows and downstream authentication integrations.
Affected Products
- User Sync – Remote User Sync plugin for WordPress (all versions ≤ 1.0.2)
- WordPress sites with the plugin installed and active
- Administrator accounts authenticated to affected WordPress installations
Discovery Timeline
- 2025-09-17 - CVE-2025-9891 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-9891
Vulnerability Analysis
The vulnerability resides in the mo_user_sync_form_handler() function within the plugin's main file mo-user-sync-main.php. This handler processes administrative form submissions but fails to validate a WordPress nonce token before executing state-changing operations. WordPress nonces exist specifically to prevent CSRF attacks by ensuring requests originate from legitimate authenticated sessions rather than forged external requests.
Without nonce validation, the handler cannot distinguish between a genuine administrator action and a request forged by an attacker. Any authenticated administrator visiting an attacker-controlled page can unknowingly trigger the plugin's form handler through embedded HTML forms or JavaScript payloads.
The attack requires user interaction, limiting exploitation to social engineering scenarios where an administrator is lured into clicking a malicious link or visiting a compromised page while logged into WordPress.
Root Cause
The root cause is missing or incorrect nonce validation on the mo_user_sync_form_handler() function. WordPress provides wp_verify_nonce() and check_admin_referer() functions for exactly this purpose, but the affected code path does not invoke them before processing form data that modifies plugin state.
Attack Vector
An attacker crafts a malicious HTML page containing an auto-submitting form or JavaScript that issues a request to the target WordPress site's admin endpoint. The request targets the plugin's form handler with parameters that trigger plugin deactivation. When an authenticated administrator visits this page, the browser automatically includes authentication cookies, causing WordPress to accept the request as legitimate. Refer to the Wordfence Vulnerability Analysis and the WordPress Plugin Code Review for the affected code location.
Detection Methods for CVE-2025-9891
Indicators of Compromise
- Unexpected deactivation of the User Sync – Remote User Sync plugin in the WordPress admin interface
- HTTP POST requests to admin endpoints containing mo_user_sync parameters without a valid _wpnonce field
- Referer headers pointing to external, untrusted domains on plugin-related admin requests
- Administrator session activity immediately following clicks on suspicious external links
Detection Strategies
- Monitor WordPress audit logs for plugin state changes correlated with administrator browsing activity
- Inspect web server access logs for POST requests to wp-admin endpoints missing nonce parameters
- Deploy a Web Application Firewall (WAF) rule to flag state-changing plugin requests lacking valid nonce tokens
- Review the WordPress Plugin Changeset to identify patched code paths for comparison
Monitoring Recommendations
- Enable WordPress activity logging plugins to track administrator actions and plugin state transitions
- Alert on any plugin activation or deactivation event outside of scheduled maintenance windows
- Correlate cross-origin referer patterns with administrator session events
- Monitor for anomalous administrator behavior following external link interactions
How to Mitigate CVE-2025-9891
Immediate Actions Required
- Update the User Sync – Remote User Sync plugin to a version newer than 1.0.2 as soon as a patched release is available
- Restrict WordPress administrator accounts to dedicated browsers or sessions that do not visit untrusted sites
- Train administrators to recognize social engineering attempts targeting authenticated sessions
- Audit currently installed plugins and remove the User Sync plugin if it is not required
Patch Information
A plugin changeset addressing the vulnerability has been published to the WordPress plugin repository. Review the WordPress Plugin Changeset for the fix implementation, which introduces nonce validation on the affected form handler. Administrators should apply the update through the WordPress plugin management interface.
Workarounds
- Deactivate and remove the User Sync – Remote User Sync plugin until a patched version is installed
- Implement a WAF rule requiring valid _wpnonce parameters on all wp-admin POST requests targeting the plugin
- Enforce SameSite=Strict cookie attributes on WordPress authentication cookies where compatible
- Require administrators to authenticate through a separate browser profile isolated from general web browsing
# Example WAF rule concept to block plugin requests missing nonce
# ModSecurity-style pseudo-rule
SecRule REQUEST_URI "@contains wp-admin" \
"chain,deny,status:403,msg:'Missing WordPress nonce on User Sync request'"
SecRule ARGS_NAMES "@rx mo_user_sync" \
"chain"
SecRule &ARGS:_wpnonce "@eq 0"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
